This package is rejected as syntactically invalid by go/parser and cmd/compile (and Go 1.4's 6g), but accepted by gccgo:

package p

type T struct{}

func F() {
	for ; T{} == T{}; {}
}

The Go spec says [emphasis added]:

A parsing ambiguity arises when a composite literal using the TypeName form of the LiteralType appears as an operand between the keyword and the opening brace of the block of an "if", "for", or "switch" statement, and the composite literal is not enclosed in parentheses, square brackets, or curly braces. In this rare case, the opening brace of the literal is erroneously parsed as the one introducing the block of statements. To resolve the ambiguity, the composite literal must appear within parentheses.

I think the intention here is that this is a gccgo parser issue for allowing TypeName composite literal between for and { without enclosing it in parentheses.

However, arguably this case isn't actually ambiguous: we've seen one semicolon after the for keyword, so we know to expect another ; before the {. And so the emphasized sentence technically doesn't apply as there's no ambiguity to resolve, and the program should be accepted.

/cc @griesemer @ianlancetaylor